This invention relates to ionizing radiation converters, such as image intensifiers, and more particularly, to x-ray image intensifier tubes.
In this specification the term "ionizing radiation" is used to denote electromagnetic radiation associated with photons having energy of at least 15 ev. Thus, X-rays, gamma rays and some ultraviolet rays are all types of ionizing radiation. The term "ionizing radiation converter" includes within its scope ionizing radiation image intensifier tubes, but is not limited to such tubes and also includes ionizing radiation detectors, for example.
The X-ray image intensifier tubes known to the applicant comprise an input clement or cathode in the form of an ionizing radiation to photoelectron converter located towards one end of the tube, an anode located towards an opposite end of the tube and intermediate focusing electrodes. The anode comprises a layer of output phosphor on a face of a transparent output window facing the cathode. In use, the cathode is illuminated by X-rays to form a primary visible image on an input phosphor layer on the cathode. Photoelectrons emitted from the cathode as a result of the illumination are accelerated and focused by the focusing electrodes so that an intensified visible output image is caused in the output phosphor layer by the impinging photoelectrons. The image is visible through the transparent output window.
It is known that the clarity of an image produced by an X-ray image intensifier tube is proportional to the product of the quantum detection efficiency (QDE) and the modulation transfer function (MTF) of the tube. The ODE is a measure of the efficiency with which on average each absorbed X-ray photon is detected, that is, made visible, by the tube at its output. The MTF is a measure of the reduction in contrast and sharpness by the tube in the output image of the spatial detail of the primary image.
As is illustrated in U.S. Pat. No. 5,144,123 to Malashanko, it has been the general trend to improve the imaging quality of the conventional X-ray tubes by improving or eliminating some of the inefficient energy conversion processes in the conventional tubes. However, the imaging quality of such conventional tubes remains limited by the MTF of the refracting electron optics, with their inherent image defects, used in all these tubes. For example, input X-rays penetrate the cathode, impinge on the output phosphor and cause unwanted background and fogging in the image caused by the photoelectrons impinging on the output phosphor.
In 1978, in a different art, that of first generation night vision apparatus, the so-called fountain tube using catadioptric electron optics was disclosed by R. Evrard. This night vision tube had a 0.5 image magnification factor, it was never commercialized, since its photon gain was limited and it could not match the photon gain of later generations of conventional night vision apparatus, which comprise electron multiplier arrangements.